style(frontend): Prettier config + one-time repo-wide format pass

Prettier 3 (pinned), printWidth 100, double quotes — chosen to match the existing
hand-formatting and minimise reflow. This commit is ONLY the mechanical format pass
(`prettier --write`) plus the config/ignore/scripts, isolated so it never pollutes a
feature diff. Verified behaviour-neutral: typecheck, eslint (0 errors), and vitest
all green before and after. See .git-blame-ignore-revs — `git blame` skips this sha.

frontend/src/components/AddToPlaylist.tsx

@@ -133,14 +133,14 @@ export default function AddToPlaylist({
             className="z-overlay glass-menu rounded-xl p-2 shadow-2xl"
             onClick={(e) => e.stopPropagation()}
           >
-            <div className="text-xs text-muted px-1.5 pb-1.5">
-              {t("playlists.addToPlaylist")}
-            </div>
-            <div ref={fade.ref} style={fade.style} className="max-h-56 overflow-y-auto no-scrollbar">
+            <div className="text-xs text-muted px-1.5 pb-1.5">{t("playlists.addToPlaylist")}</div>
+            <div
+              ref={fade.ref}
+              style={fade.style}
+              className="max-h-56 overflow-y-auto no-scrollbar"
+            >
               {lists.length === 0 && !membership.isLoading && (
-                <div className="text-xs text-muted px-1.5 py-2">
-                  {t("playlists.noneYet")}
-                </div>
+                <div className="text-xs text-muted px-1.5 py-2">{t("playlists.noneYet")}</div>
               )}
               {lists.map((pl) => (
                 <button
@@ -151,16 +151,12 @@ export default function AddToPlaylist({
                 >
                   <span
                     className={`grid place-items-center w-4 h-4 rounded border ${
-                      pl.has_video
-                        ? "bg-accent border-accent text-accent-fg"
-                        : "border-border"
+                      pl.has_video ? "bg-accent border-accent text-accent-fg" : "border-border"
                     }`}
                   >
                     {pl.has_video && <Check className="w-3 h-3" />}
                   </span>
-                  <span className="truncate flex-1 text-left">
-                    {playlistName(pl, t)}
-                  </span>
+                  <span className="truncate flex-1 text-left">{playlistName(pl, t)}</span>
                   {(pl.source === "youtube" || pl.yt_playlist_id) && (
                     <Youtube className="w-3 h-3 shrink-0 text-muted" />
                   )}
@@ -180,7 +176,7 @@ export default function AddToPlaylist({
               />
             </form>
           </div>,
-          document.body
+          document.body,
         )}
     </>
   );